Out-of-plane micro-electromechanical systems (MEMS) accelerometers typically use parallel plate capacitance pickoff sensing (sense) for open-loop devices and parallel plate capacitance rebalance (torque) for closed-loop devices. Both types of sensors are composed of two parallel plates separated by a narrow gap and having a voltage applied across them. In either case, the functionality of the device derives from the variation in capacitance that occurs when the distance across the gap changes under acceleration.
Because capacitance varies inversely as a function of the square of the gap (C=εA/x2, where C=capacitance and x=the length of the gap), the response of a parallel plate-type accelerometer is inherently nonlinear. Due to the nonlinear dependence, variation in the gap distance from environmental influences, such as thermal deformation or vibration, are difficult to remove by calibration.